Exciton Biexciton Research Articles

Exciton–biexciton quantum coherence and polaritonic stop‐band transparency in CuCl

A coherently driven exciton–biexciton transition in CuCl enables one to propagate a probe light beam within the exciton-polariton stop-band where radiation is otherwise completely reflected. The stop-band transparency window can be controlled via the pump beam frequency and intensity. The phenomenon is reminiscent of quantum coherence effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystal via a frequency and wave-vector selective polaritonic mechanism. Both a free standing slab and a microcavity configuration are theoretically studied. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Symmetry aspects of exciton–exciton interactions and biexciton transitions in wurtzite semiconductor structures

AbstractThe inelastic collision of two excitons or the optical decay of a biexciton can induce a transition into an exciton and a photon. From the symmetry point of view, we have studied the states of free and bound biexcitons in bulk GaN with the wurtzite structure and in GaN/AlN heterostructures. Due to the antisymmetry properties arising from the exchange of identical fermions, the ground state of the free Γ biexcitons involving two holes from the same valence band is totally symmetric with respect to the symmetry group of the structure. We established the optical selection rules as well as the possible channels for the processes mentioned above and showed that, whatever is the symmetry of a biexciton, there are at least two channels for its optical recombination. That suggests fast recombination processes. If one (several) phonon(s) is (are) involved in a process, it is always possible to connect by an optical transition any initial state to any final one. An electric field applied parallel to the growth axis lowers the symmetry of superlattices to that of quantum wells. All the results remain valid for any bulk semiconductor with the wurtzite structure and for any wurtzite‐based heterostructure with identical cations or anions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence study of ZnO nanorods epitaxially grown on sapphire (112̄0) substrates

ZnO nanorods were synthesized on sapphire (112̄0) substrates by metalorganic vapor deposition. The rods exhibited better crystalline and optical properties than those of ZnO rods formed on sapphire (0001) substrates. The emission due to biexcitons is persistent up to ∼200 K, indicating potential for applications in biexciton-based nanoscale short-wavelength light-emitting photonic devices. The exciton–biexciton energy separation is independent of sample temperature. The band edge emission peak at room temperature is a mixture of free exciton and impurity-related transitions.

Time domain investigation on excitonic spectral diffusion in CdSe quantum dots grown on vicinal surface GaAs substrates

Two monolayers of CdSe sandwiched by ZnSe layers were grown by molecular-beam epitaxy on a vicinal GaAs surface substrate. Small ensembles of these self-assembled quantum dots (QD) have been studied at low temperatures using micro-photoluminescence spectroscopy. We observe reversible spectral diffusion in individual QD luminescence lines. The energy shift is negligibly small at low optical excitation power and amount to about 0.7meV under higher excitation (0.6kW/cm2). Because the energy shift is found to be significantly smaller than the exciton–biexciton splitting of CdSe QDs even under high excitation, the conventional spectral filtering could be used to effectively select the exciton-related emission from these QDs.

Direct two-photon creation of a biexciton

The nonlinear absorption coefficient for the two-photon creation of biexciton in CuCl is calculated in the approximation of one intermediate, optically active excitonic state. The two-band model with realistic band structure at the band extrema is considered. The exciton–biexciton dipole transition momentum matrix element is expressed by the functional of the exciton and biexciton envelopes. In the numerical calculations the envelope of the Hylleraas–Ore type, modified by Brinkman et al., is used. Our calculated value of the absorption coefficient is about two orders of magnitude smaller than that estimated previously by Hanamura, and agrees better with the experimental value reported by Gale and Mysyrowicz.

Shape‐dependent properties of self‐organized quantum dots: Few‐particle states and exciton‐phonon coupling

AbstractThe electronic and optical properties of self‐organized InAs/GaAs quantum dots are investigated in view of the actual structural properties. Focussing on the shape we demonstrate theoretically a strong impact of the varying strain distribution on the electronic and, in particular, the exciton properites in such quantum dots. The inhomogeneous strain, typical for self‐organized quantum dots, lowers the symmetry and increases the local charge density. Resonant Raman experiments on pyramidal InAs/GaAs quantum dots show an enhance exciton–LO‐phonon coupling reflecting the strain‐induced local charge density. Experiments on single InAs/GaAs quantum dots demonstrate an unexpected strong impact of the structural properties on few‐particle complexes on the example of the biexciton complex. Anti‐binding biexciton complexes are demonstrated. The apparent correlation of the biexciton binding energy and exciton transition energy, with a transition from binding to anti‐binding at ∼1.24 eV, is attributed to a decreasing exciton localization, resulting from the finite barrier height. The Coulomb interaction with delocalized states is negligible for the energy of the localized states.

Single‐photon and photon‐pair emission from CdSe/Zn(S,Se) quantum dots

AbstractWe report on the generation of triggered single photons and photon pairs relying on pulsed optical excitation of epitaxially grown self‐assembled CdSe quantum dots (QDs). Single‐photon emission is studied on individual CdSe/Zn(S,Se) QDs for temperatures up to 200 K. At low temperatures nearly perfect single‐photon emission is achieved whereas at higher temperatures an increasing multi‐photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring QDs is observed. The multi‐photon emission probability of a bare QD (background subtracted) is strongly suppressed. Furthermore, the polarization cross correlation of the biexciton–exciton cascade has been investigated in a CdSe/ZnSe QD at low temperatures. A strong polarization correlation of the emitted photon pairs of ∼74% is observed in a linear detection basis.

Confinement-enhanced biexciton binding energy in ZnO/ZnMgO multiple quantum wells

By employing a nanosecond pump-probe method, biexciton formation process was investigated in ZnO/Zn1−xMgxO (x=0.26) multiple quantum wells (MQWs) grown on ScAlMgO4 substrate by laser molecular-beam epitaxy. Bleaching of absorption due to the saturation of excitonic states, and induced absorption related to the exciton–biexciton transition were observed in their spectra. It is demonstrated that the pump-probe method allows us to precisely determine binding energies of exciton complexes even applied to the semiconductor quantum structures where the localization effect are not negligible. This is because a transition from free-excitonic states to free-biexcitonic states is involved in the induced absorption process. The biexciton binding energy is a monotonically decreasing function of well width (Lw). For the MQWs with Lw smaller than 2.5 nm, the biexciton binding energy is larger than 25 meV, comparable to the thermal energy of room temperature.

Optical coherent control and phase shift of excitonic and biexcitonic four-wave-mixing polarization

The optical coherent control of the exciton–biexciton system is studied in four-wave-mixing (FWM) experiments on a ZnSe single-quantum well. Coherent control is achieved by applying a pair of phase-locked laser pulses which produce an FWM signal in combination with an additional third pulse. Oscillations as a function of the interpulse delay time are observed in the FWM signal at the energetic position of the exciton and biexciton resonance, respectively. A phase shift between these oscillations is found for certain orders of arrival of the excitation pulses at the sample. This clearly demonstrates that the coherent control acts in different ways: if the phase shift is observed the FWM polarization is coherently controlled. Otherwise, the coherent control just involves the polarization induced by the two phase-locked pulses.

Coherent exciton–biexciton dynamics in GaN

Spectrally resolved and time-integrated four-wave mixing are used to measure the polarization dependence of biexcitonic signals and quantum beats between two-$A$-exciton ${(X}_{A}{X}_{A}^{*})$ and A-biexciton ${(X}_{A}{X}_{A})$ states in a high-quality GaN epilayer. Mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between ${X}_{A}{X}_{A}^{*}$ and ${X}_{A}{X}_{A};$ the second period corresponds to beating between A excitons ${(X}_{A})$ and donor bound excitons ${(D}^{0}X).$ We also measure the polarization-dependent B-biexciton ${(X}_{B}{X}_{B})$ signal. The effective masses for the A and B holes are deduced from the binding energy.

Charged excitons in single CdTe quantum dots

Optical emission from individual n-type modulation-doped CdTe quantum dots (QDs) is presented. Exciton, biexciton and negatively charged exciton emission lines are identified in the micro-photoluminescence spectra. All these few-particle species are exposed to randomly fluctuating electric fields attributed to charges trapped in the vicinity of the QD. The resulting quantum-confined Stark effect, characteristic for each QD, is analyzed in detail. The correlation between the emission energies and the integrated intensities of the neutral and charged exciton permit one to confirm the identification of the charged species. The binding energies of the biexciton and charged exciton are found to sharply decrease with the magnitude of the local electric field.

Boson characteristics of the exciton–biexciton system in a GaAs/AlAs type-II superlattice

We report on the appearance of Bose–Einstein statistics of the exciton–biexciton system in a (GaAs) 12/(AlAs) 12 type-II superlattice. The density relation of the exciton and biexciton is estimated from time-resolved photoluminescence spectra at various excitation powers and bath temperatures. The long lifetime of the type-II exciton, e.g., 2 μs at 5 K, enables us to obtain the precise information of the density relation. In a relatively low exciton-density region, the biexciton density obeys a well-known square law. At an exciton density around 1×10 10 cm −2, the biexciton density suddenly increases with a threshold-like nature. This behavior, which is realized at temperatures up to 8 K, can be understood from Bose–Einstein statistics.

Spectral signatures of χ^(5) processes in four-wave mixing of homogeneously broadened excitons

The influence of fifth-order coherences on the spectrally resolved four-wave mixing response of predominantly homogeneously broadened quasi-two-dimensional excitons is studied. Fifth-order signatures are discussed as a function of spectral position and excitation polarization. An exciton–biexciton beating for positive delay times is the dominant effect, which is pronounced at the exciton–biexciton transition for collinearly polarized excitation and at the exciton transition for cross-linearly polarized excitation. For negative delay times and collinearly polarized excitation a pronounced exciton–biexciton beating at the exciton resonance is observed that is vanishing for long negative delays owing to the faster dephasing in the two-exciton continuum compared with the bound biexciton state. These results are in qualitative agreement with microscopic model calculations that include the coherent dynamics of one- and two-exciton resonances up to the fifth order in the optical field.

Biexciton binding energy and exciton–LO-phonon scattering in ZnSe quantum wires

The dependence of the biexciton binding energy and of the exciton--LO-phonon scattering rate on the wire width is investigated in wet-etched ZnSe quantum wires by temperature-dependent transient four-wave mixing. We observe an increase of the biexciton binding energy with decreasing wire width, reaching an enhancement of about 30% in the smallest wire structure. In addition, we find a decrease of the exciton--LO-phonon scattering rate with decreasing wire size, which is discussed with consideration of the reduced polarity of the exciton wave function and the modified density of final states in narrow wire structures.

Effects of Electronic Fourth-Order Correlations on the Pump-Probe Response of Semiconductors

The pump-and-probe response of a ZnSe quantum well is investigated. In this particular sample the binding energy far exceeds both the exciton damping and the inhomogeneous broadening. A comparison between experimental data and model calculations shows that with co-circularly polarized beams the nonlinear mean-field contributions dominate those induced by four-particle correlations. In contrast, only the four-particle correlations contribute to the signal in counter-circular configuration. For this configuration, the exciton–biexciton interaction dominates the response for resonant excitation of the exciton. The occurrence of exciton–biexciton transitions leads to a renormalization of the period of coherent oscillations at negative pulse delay.

Biexcitons in CdMnTe/CdTe/CdMgTe single quantum well

We have performed two-pulse and three-pulse time-integrated as well as spectrally resolved four-wave-mixing measurements on a single CdMnTe/CdTe/CdMgTe quantum well. The polarization dependence of both four-wave-mixing measurements can be explained well by the five-level model of the exciton–biexciton system including two-biexciton states with parallel spin and anti-parallel spin orientations.

Polariton–biexciton transitions in a ZnSe-based microcavity

The optical third-order nonlinearity of a ZnSe-based microcavity is investigated by the pump-and-probe method. In the specially designed non-monolithic sample the biexciton binding energy exceeds all damping constants and the normal-mode splitting between exciton and cavity photon. For counter-circular polarized beams the nonlinear response exhibits strong oscillatory structures in the spectral vicinity of the polariton–biexciton transition. Comparison to model calculations shows that in this case the coherent nonlinearity is completely dominated by biexciton–exciton interactions beyond the Hartree–Fock approximation.

Beat structure in spectrally resolved four-wave mixing under crosslinear polarization in GaAs quantum wells

The polarization dependence of beat structure in spectrally resolved four-wave mixing was investigated on 50Å GaAs multiple quantum wells. Under crosslinear polarization we observed a beating structure at higher energy region of the main spectral peak due to biexciton–exciton transition. The beat has a period of 0.95 ps and is constructive at delay time T=0ps, which suggests the corresponding double Feynman diagrams to be of the same type. By shortening the central wavelength of the sub-ps laser, we observed the distinctive peak corresponding to the beat period. Even in GaAs system, which is generally believed to have a small biexcitonic effect, there are two effective 2-exciton states with well-defined energies which are required to describe the third-order optical nonlinearly.

Temperature-dependent line widths of single excitons and biexcitons

The coupling of single zero-dimensional excitons and biexcitons to acoustic and optical phonons is studied in epitaxially grown Zn 1− x Cd x Se/ZnSe quantum structures. From the temperature-dependent line width of the exciton photoluminescence (PL) an enhanced coupling to acoustic phonons is found which we explain by the quantum confinement. The LO-phonon coupling is reduced compared to CdSe bulk material. The temperature dependence of the line width of the biexciton PL is similar, indicating a similar coupling strength of the exciton and the exciton–biexciton transition to the crystal lattice.

Exciton and biexciton correlations for weakly confined semiconductor quantum wires

The contributions of excitons and biexcitons to the four-wave mixing (FWM) signal are calculated for semiconductor quantum wells with an additional weak wire confinement. In contrast to the giant oscillator model, in our description the biexciton is formed from two singlet excitons by the Coulomb interactions. The matrix elements of the repulsive and attractive interactions between two excitons are calculated explicitly using 2D quantum well wave functions and quantized center-of-mass envelop functions. By calculating the excitonic polarization in the FWM direction, we obtain coherent exciton–biexciton quantum beats as function of the delay between the two pulses and of their intensity. Quantum beat oscillations are found not only on the rising side of the FWM-signal as in spatially homogeneous χ 3-theories, but also on the falling side, as it is observed experimentally. With increasing intensities the quantum beats develop into irregular signals.